The present invention relates in general to electronic oscillator circuitry, and in particular to an oscillator circuit for a piezoelectric crystal which compensates for and effectively cancels out any shunt capacitance associated with the piezoelectric crystal thereby improving the ability of the crystal to continue to oscillate although contacted by a viscous, fluid or solid medium and to enable more accurate measurements of characteristics (e.g. density and viscosity) of the medium.
Piezoelectric crystals can be used to sense properties a viscous medium or changes in the properties thereof when the viscous medium is brought into contact with a surface of the piezoelectric crystal when the crystal is electrically activated into resonance (see e.g. U.S. Pat. No. 5,201,215 which is incorporated herein by reference). The electrical activation of the crystal can be performed by placing the crystal into an oscillator circuit which generally operates at a frequency corresponding to an odd multiple of one-half of an acoustic wavelength of the crystal so that at resonance a standing shear wave is generated across the thickness of the crystal. Contact of a surface of the crystal by the viscous medium results in a decrease in the oscillation frequency of the crystal and a damping of the resonance which can be used to ascertain characteristic properties (e.g. viscosity and density) of the medium. To sustain oscillation of the crystal during contact with the viscous medium, an electrical circuit driving the crystal must be capable of operating over a wide dynamic range of resonator loss, Rm. This dynamic range of Rm can be over several orders of magnitude. A shunt capacitance associated with the crystal detrimentally affects the performance of the oscillator circuit by making operation at a series resonance, fs, difficult and by limiting the dynamic loss range that the oscillator can tolerate. The series resonance, fs, is defined herein as the frequency where an inductance Lm and a capacitance Cm of a motional arm of the piezoelectric crystal are in resonance. At this frequency fs, if a shunt capacitance, C0, and a parasitic capacitance, Cp, associated with the piezoelectric crystal are both small, then the crystal impedance is real and the oscillator circuit of the present invention is a real impedance phase oscillator which operates at or near a zero degree impedance phase angle.
A Lever oscillator circuit that has previously been used to drive a quartz crystal resonator for fluid sensing applications is disclosed in U.S. Pat. No. 5,416,448 which is incorporated herein by reference. The present invention represents an improvement over this prior art oscillator circuit by providing a circuit design which more accurately operates at fs with a large resonator shunt capacitance and which does not require automatic level control circuitry to determine Rm. The present invention provides for operation of a piezoelectric crystal over a wide range of resonator loss by effectively cancelling out the shunt capacitance of the resonator via an active circuit and a dummy capacitance (i.e. a reference capacitor). The oscillator circuit of the present invention has a relatively small component count compared to prior circuits employing automatic gain control circuitry as disclosed in U.S. Pat. No. 5,416,448, thereby permitting the oscillator circuit of the present invention to be formed as a discrete circuit or as an integrated circuit (IC) with a reduced size, a reduced power consumption and a reduced manufacturing cost.
The present invention relates to a oscillator circuit for operating a piezoelectric crystal at a frequency of oscillation near a series resonance of the crystal. The oscillator circuit comprises a tuned gain stage which includes a limiting amplifier located in a feedback loop about a transistor, with a non-inverting output of the transistor being connected to drive the crystal at the frequency of oscillation, and with an inverting output of the transistor being connected to a resonant tank circuit to suppress oscillation of the crystal at frequencies other than near the series resonance. The oscillator circuit also includes a compensation circuit that effectively cancels any shunt capacitance associated with the piezoelectric crystal. The compensation circuit receives an input from the output of the limiting amplifier, with the compensation circuit providing an output that is connected to the crystal at the non-inverting transistor output. The compensation circuit is advantageous in that the oscillator circuit operating at a zero-impedance phase will oscillate at the series resonance frequency, fs, of the piezoelectric crystal independent of any shunt capacitance associated with the crystal thereby effectively cancelling out any effect on the frequency of oscillation due to the shunt capacitance. Thus, the compensation circuit acts to provide all the current flow through the shunt capacitance so that the oscillator circuit need not supply this current flow. The shunt capacitance associated with the crystal is defined herein as being the sum of a static capacitance, C0, which arises from internal fields across the crystal and any parasitic capacitance, Cp, due to packaging of the crystal and wiring thereto.
The resonant tank preferably comprises a low-Q tank circuit that is used to connect the oscillator circuit to a power supply. This resonant tank peaks the gain of the tuned gain stage in a desired frequency range near the series resonance, fs, and helps to reject unwanted modes of oscillation. The tuned gain stage can further include a current source provided between the non-inverting transistor output and an electrical ground for direct-current (dc) biasing of the transistor.
The limiting amplifier is preferably a differential amplifier with one input connected to an inverting output of the transistor, and with another input being electrically grounded at the frequency of oscillation. The output of the limiting amplifier is connected to the input of the transistor to provide positive feedback for oscillation. The limiting amplifier also preferably provides a frequency output signal that is representative of the frequency of oscillation of the crystal, and a dc voltage output signal that is representative of a resonator loss component, Rm, of the crystal.
The compensation circuit further includes a current mirror which provides dc and alternating-current (ac) current components at the non-inverting output of the transistor, with the ac current component being equal in magnitude and phase to a current flow through the shunt capacitance associated with the crystal at the frequency of oscillation. This ac current component effectively cancels out any shunt of the crystal due to the shunt capacitance by providing all the current flow through the shunt capacitance independent of the gain stage. The value of this ac current component is determined by a reference capacitor located in the compensation circuit. The reference capacitor is selected to have a value of capacitance that is equal to the shunt capacitance associated with the crystal. This can be done, for example, by providing the reference capacitor as a variable capacitor and tuning the variable capacitor during a calibration of the oscillator circuit.
The oscillator circuit of the present invention can operate the crystal with one side electrically grounded, and with a frequency of oscillation that is generally in the range of 1-100 MHz. This circuit can be formed as a discrete circuit using discrete components, or alternately can be formed, at least in part, as an integrated circuit (IC). Embodiments of the oscillator circuit of the present invention can be formed using either bipolar transistors or field-effect transistors (FETs). When the oscillator circuit is formed as an IC, certain components (e.g. the reference capacitor and tank circuit) can be outboard to the IC to allow adaptation of the IC to many different types of applications, or to different types and sizes of piezoelectric crystals.
The present invention also relates to an oscillator circuit to provide for oscillation of a piezoelectric crystal, with the oscillator circuit comprising a transistor having a limiting amplifier connected in a positive feedback configuration between an inverting output of the transistor and an input thereof, and with the oscillator circuit further including a compensation circuit for effectively cancelling out the shunt capacitance associated with the crystal. The piezoelectric crystal is connected to a non-inverting output of the transistor with the other side of the crystal being electrically grounded. The compensation circuit provides an ac current component at the non-inverting output which is equal in magnitude and phase to a current flowing through the shunt capacitance. As a result, the compensation circuit completely supplies any required current flow through the shunt capacitance; and this effectively cancels out any effect of the shunt capacitance on the oscillator circuit, or on a frequency of oscillation, f, of the crystal. Effectively cancelling out the shunt capacitance also allows the piezoelectric crystal to oscillate over a wide range of damping when placed into contact with a viscous, fluid or solid medium.
In this oscillator circuit, the limiting amplifier comprises a differential amplifier with a first amplifier input being capacitively coupled to the inverting output of the transistor, and with a second amplifier input being capacitively coupled to an electrical ground. The limiting amplifier is also preferably configured to provide a frequency output signal to indicate the frequency of oscillation of the crystal, and a dc voltage output signal to indicate a resonator loss component, Rm, within the crystal.
The compensation circuit comprises a current mirror for providing the ac current component. The magnitude and phase of the ac current component provided by the current mirror is determined using a reference capacitor connected to the current mirror through another transistor whose input is connected to the output of the limiting amplifier. The reference capacitor can be adjustable (i.e. a variable capacitor) to allow for a precise adjustment of the magnitude of the ac current from the current mirror in order to cancel out the loading due to the shunt capacitance associated with the crystal.
The present invention further relates to an oscillator circuit for operating a piezoelectric crystal at a frequency of oscillation near a series resonance, fs. This oscillator circuit comprises a first transistor having a limiting amplifier connected in a positive feedback configuration between an inverting output of the first transistor and an input thereof, with a non-inverting output of the first transistor being further connected to one side of the piezoelectric crystal, and with the other side of the piezoelectric crystal being electrically grounded. In this circuit, a current mirror is connected between an inverting output of a second transistor and the non-inverting output of the first transistor, with the second transistor further having an input connected to the first transistor input. The current mirror generates an ac current component at the frequency of oscillation that is sufficient to supply any current flowing through a shunt capacitance associated with the piezoelectric crystal. This effectively cancels out the shunt capacitance, and allows the crystal to operate over a wide dynamic range of motional resistance, Rm, which changes in response to contact of an exposed surface of the crystal by a viscous, fluid or solid medium. The magnitude of the ac current component provided by the current mirror can be determined by a reference capacitor which can be a variable capacitor.
Current sources are preferably provided within the oscillator circuit at the non-inverting outputs of each transistor (i.e. in parallel with the piezoelectric crystal, and in parallel with the reference capacitor) to establish dc bias levels for these transistors. The functional elements of the oscillator circuit (e.g. the first and second transistors, the limiting amplifier and the current mirror) can be optionally formed as an IC.
The present invention also relates to an oscillator circuit for operating a piezoelectric crystal at a frequency of oscillation near a series resonance (i.e. a series resonance frequency). This oscillator circuit comprises a first transistor having an input, an inverting output and a non-inverting output, with the piezoelectric crystal being connected between the non-inverting output and a ground connection to a power supply, and with the inverting output of the first transistor being connected to the power supply through a resonant tank circuit. A first current source is connected between the non-inverting output of the first transistor and the ground connection in parallel with the piezoelectric crystal; and a second current source is connected between the non-inverting output of a second transistor and the ground connection in parallel with a reference capacitor. This second transistor has its input connected to the input of the first transistor and further connected to the output of a limiting amplifier which is located in a feedback loop between the inverting output of the first transistor and the inputs of both transistors to provide the positive feedback necessary for oscillation of the crystal.
A current mirror is also used in the oscillator circuit and is connected between the inverting output of the second transistor and the power supply to generate an ac current component at the frequency of oscillation and provide this ac current component to the non-inverting output of the first transistor. The magnitude of the ac current component is controlled and determined by the reference capacitor so that the magnitude of the ac current component is equal to that of an ac current flowing through a shunt capacitance associated with the piezoelectric crystal. As a result, the shunt capacitance on the oscillator circuit is effectively cancelled out.